Generally, in IEEE 802.16 based wireless mobile communication system, a mobile station basically performs handover ranging for synchronization with a target base station. Yet, in order to selectively minimize handover latency, it may directly send a ranging request (RNG-REQ) message by omitting a process for matching uplink synchronization through transmission of CDMA ranging code. This is called fast ranging.
FIG. 1 is a flowchart of signals for a handover process using fast ranging according to a related art.
Referring to FIG. 1, a mobile station sends a handover request (MOB_MSHO-REQ) message to a serving base station (SBS: serving BS) [S110]. The serving base station transmits a handover request HO-REQ to a target base station (TBS: target BS) [S120] and then receives a handover response (HO-RSP) from the target base station [s130].
The serving base station informs the mobile station of a time for transmitting fast ranging information elements (Fast_Ranging_IE) of target base stations using an action time field of a handover response (MOB_BSHO-RSP) [S140].
Thus, if a non-zero time is assigned through the action time field, the mobile station may perform fast ranging on the target base station after the assigned time. Therefore, the mobile station may transmit a handover indication (MOB_HO_IND) message to the serving base station [S150].
Subsequently, the mobile station successfully receives a fast ranging information element (Fast_Ranging_IE) without additional setup, and is then able to receive uplink (UL) allocation for transmitting a ranging request (RNG-REQ) [S160].
Meanwhile, the serving base station calculates the above-mentioned action time using a handover (HO) readiness timer and a base station (BS) switching timer. In this case, the mobile station may estimate time alignment (hereinafter abbreviated TA) information using an uplink resource parameter with the serving base station, a preamble reception time difference between the serving base station and the target base station and the like.
The mobile station finally transmits a ranging request (RNG-REQ) to the target base station using the estimated TA information [S170]. If the mobile station receives a ranging response (RNG-RSP) from the target base station [S180], the mobile station may perform communication with the target base station on user data.
The above-described fast ranging procedure is performed while the mobile station receives necessary ranging parameters through a pre-scanning process and a neighbor advertisement (MOB_NBR-ADV) message receiving process.
FIG. 2 is a flowchart of signals for an initial ranging process in case that the fast ranging shown in FIG. 1 fails.
Referring to FIG. 2, a mobile station sends a handover request (MOB_MSHO-REQ) message to a serving base station [S210]. The serving base station transmits a handover request (HO-REQ) to a target base station [S220] and then receives a handover response (HO-RSP) from the target base station [S230].
The serving base station informs the mobile station of a time for transmitting fast ranging information elements (Fast_Ranging_IE) of target base stations using an action time field of a handover response (MOB_BSHO-RSP) [S240]. Accordingly, the mobile station transmits a handover indication (MOB_HO_IND) message to the serving base station [S250]. Subsequently, the mobile station successfully receives a fast ranging information element (Fast_Ranging_IE) without additional setup [S260] and is then able to receive uplink (UL) allocation for transmitting a ranging request (RNG-REQ).
If the ranging request (RNG-REQ) or ranging response (RNG-RSP) message is lost in the course of performing the fast ranging [S271], as shown in FIG. 2, the mobile station should perform initial ranging with CDMA code instead of performing the fast ranging [S272, S273, S274, S280]. From FIG. 2, it can be observed that latency of 50 ms may be generated from this process.
If a mobile station fails to correctly transmit/receive a message to/from a target base station, ranging parameters may be regarded as set not to be matched. Therefore, the ranging parameters need to be reset through initial ranging. The loss of the ranging request (RNG-REQ) or the ranging response (RNG-RSP) is generated if time synchronization or time alignment (TA) is considerably mismatched or channel estimation between the mobile station and the target base station is mismatched (e.g., a power control is mismatched). Since mobile stations performing handover are mostly located at cell edge, the possibility of the message loss is very high. And, this may raise the corresponding handover latency and may cause ongoing problems.
Yet, IEEE 802.16m system requires handover latency within 30 ms.
FIG. 3 is a diagram for handover latency in case that fast ranging shown in FIG. 1 is successful.
FIG. 3 shows an example of an optimized handover scenario using fast ranging based on a frame structure currently proposed by IEEE 802.16m system. This scenario indicates a situation after a mobile station has sent a last handover indication (MOB_HO-IND) message to a serving base station. In this case, as there is no loss of ranging request (RNG-REQ) or ranging response (RNG-RSP), handover latency of 25 ms is generated.
FIG. 4 is a diagram for handover latency in case that the fast ranging shown in FIG. 2 fails.
A scenario shown in FIG. 4 indicates a situation after a mobile station has sent a last handover indication (MOB_HO-IND) message to a serving base station.
Referring to FIG. 4, in case that loss of ranging request (RNG-REQ) or ranging response (RNG-RSP) is generated, CDMA-based initial ranging should be performed. In this case, as handover latency of 60 ms including a timer T3 is added, total 80 ms handover (HO) interruption time may be generated. For a continuously moving mobile station, propagation delay and time synchronization or power readjustment with a serving or target base station are necessary. In particular, TA mismatch or improper power may bring a loss of a control message and my cause considerably latency in performing fast ranging.